Surfaces of the type described above serve for a reduction in turbulent exchange of momentum between the flow and the wall at the surface. Inasmuch as this is achieved, a reduction in the wall friction appears.
A surface of the above-mentioned type is described by M. Bruse, D. W. Bechert, J. G. T. van der Hoeven, W. Hage and G. Hoppe: Experiments with conventional and with novel adjustable drag-reducing surfaces, Int. Conf. on Near-Wall Turbulent Flows, Tempe, Ariz., USA, 15th to 18th Mar. 1993, Proceedings published by ELSEVIER, Amsterdam. In this case, the ribs are shaped like straight lines with respect to their cross section. At a rib height of 50% of the lateral distance between the ribs, a reduction in the wall friction of up to 10% is obtained provided that the lateral distance between ribs is optimally adjusted to the individual flow. Nevertheless, this well-known surface is unsuitable for technological applications, for example on airplanes, as ribs with straight line cross sections are difficult to produce and do not offer sufficient stability.
From EP 0 205 289 A 1, various surfaces for a wall subject to a turbulent flow showing a main direction of flow are known. In every case, the surface comprises a multitude of ribs which directly border each other and which are arranged in the main direction of flow. With wedge-shaped rib structures, as described in the older European patent applications, the lateral distance between the ribs and the height of the ribs are equal.
Although EP 0 205 289 A 1 claims that the surfaces described could result in a reduction in wall friction of up to 10% and more, there is no solid verification for this claim.
For a long time, wall friction measurements were only possible with a measurement accuracy of .+-.2%. Only since the construction of a special test installation, the Berlin Oil Channel, measurement accuracies of .+-.0.3% have been able to be obtained. [D. W. Bechert, G. Hoppe, J. G. T. van der Hoeven and R. Makris: The Berlin oil channel for drag reduction research, Exp. in Fluids 12, 251 to 260 (1992)]. By means of the utilization of oil as a fluid, flow measurements could be carried out on larger ribs. This eases the production of reproducible test surfaces. Furthermore, the measurement accuracy for wall friction measurement has been modified by a comparison in wall friction forces using a smooth reference surface as opposed to absolute wall friction measurements. An examination of the surface known from the EP 0 205 289 A 1 with wedge-shaped ribs showed a maximum reduction in the wall friction of approximately 5%. A reduction in the wall friction by 10% and more mentioned in the older European patent application could not be confirmed.